The present invention relates to a process for preparing L-threonine which comprises causing D-threonine-aldolase, or D-threonine-aldolase and L-allothreonine-aldolase to act on a solution containing at least DL-threonine.
More particularly, the present invention relates to a process for preparing L-threonine which comprises causing D-threonine-aldolase to act on DL-threonine or causing D-threoninealdolase and L-allothreonine-aldolase to act on a mixture of DL-threonine and DL-allothreonine thereby obtaining L-threonine from DL-threonine or the mixture of DL-threonine and DL-allothreonine by decomposing D-threonine, D-allothreonine and L-allothreonine asymmetrically into glycine and acetaldehyde.
L-threonine is one of the essential amino acids for human and animals, and because of the relatively small content thereof in various animal and plant proteins, the potential demand for L-threonine as one of the additives into foods and feeds is large enough. Hitherto, L-threonine has been produced by a method of extraction from natural materials or a method of fermentation of natural materials, however, due to the relatively high cost of L-threonine produced by such a method, a process for preparing thereof at a lower cost has been required.
On the other hand, although L-threonine is easily synthesizable while using glycine, etc., D-threonine is by-produced in the same amount as that of L-threonine with the simultaneous formation of D-allothreonine and L-allothreonine as their DL-isomer. Accordingly, the steps of isolating and pruifying L-threonine from the reaction products are extremely complicated. The yield of L-threonine is low and the price of L-threonine produced by synthetic process is very high. For instance, as an optical resolution method of DL-threonine, the methods disclosed in Bull.Soc.Chim., Vol. 20, page 903(1953) and ibid., Vol. 23, page 447(1956) have been known, however, these methods for optical resolution of DL-threonine are extremely troublesome and only give L-threonine in a low yield. In addition, for removing allothreonines, a very troublesome and inefficient method is inevitably used such as the method disclosed in Japanese Patent Publication No. 36-19562(1961) wherein bis(acetaldehyde)threonine copper is used, or the method disclosed in U.S. Pat. No. 2,461,847 wherein allothreonine and threonine are converted into their sodium salts in ethanol by using sodium ethylate, and the salts are separated by utilizing the difference between their solubilities.
Besides, there have been demerits in the synthetic process that there are scarcely any demand for D-threonine and allothreonine in the market, and that the racemization of D-threonine into L-threonine is not so easily effected.
As a result of the present inventors' efforts in studying for developing a process for preparing L-threonine at a low cost by utilizing enzymatic reactions while dissolving the technical problems, the present inventors have found a novel enzyme which catalyzes D-threonine and also D-allothreonine to convert them into glycine and acetaldehyde, and termed the enzyme D-threonine-aldolase. Namely, the present inventors have further found that in the case where D-threonine-aldolase is brought into action on the product of the synthesis, i.e., DL-threonine, or in the case where D-threonine-aldolase in combination with L-allothreonine-aldolase is brought into action on the more complicated products of synthesis, i.e., a mixture of DL-threonine and DL-allothreonine, L-threonine together with the useful decomposition-product, i.e., glycine and acetaldehyde are obtained. As the result, the preparation of L-threonine is easily carried out and the by-products are utilizable while dissolving all the problems shown above.
In an aspect of the present invention, there is provided a process for preparing L-threonine which comprises causing D-threonine-aldolase or D-threonine-aldolase and L-allothreonine-aldolase to act on a solution containing at least DL-threonine. More particularly, there is provided a process for preparing L-threonine which comprises causing D-threonine-aldolase to act on DL-threonine or causing D-threonine-aldolase and L-allothreonine-aldolase to act on a mixture of DL-threonine and DL-allothreonine thereby obtaining L-threonine from DL-threonine or the mixture of DL-threonine and DL-allothreonine.
Namely, the present invention relates to a process for obtaining L-threonine from DL-threonine or a mixture of DL-threonine and DL-allothreonine, comprising the step of catalyzing DL-threonine contained in an aqueous solution with D-threonine-aldolase or catalyzing a mixture of DL-threonine and DL-allothreonine contained in an aqueous solution with a mixture of the D-threonine-aldolase and L-allothreonine-aldolase.
The D-threonine-aldolase according to the present invention is a novel enzyme which decomposes D-threonine into glycine and acetaldehyde and also catalyzes D-allothreonine to decompose thereof into glycine and acetaldehyde. An enzyme produced by a strain of Alcaligenes faecalis, IFO 12669 (deposited in Institute for Fermentation Osaka, Japan), an enzyme produced by a strain of Pseudomonas DK-2, deposited in Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan under a deposite number of FERM-P No. 6200, and an enzyme produced by a strain of Arthrobacter DK-19, also deposited in the latter Institute under a deposite number of FERM-P No. 6201 respectively possess an activity of deocmposing D-threonine and D-allothreonine and accordingly, each of them can be used according to the present invention.
The bacteriological properties of the strain of Pseudomonas DK-2 (FERM-P No. 6200) and the strain of Arthrobacter DK-19 (FERM-P No. 6201) are shown below.
______________________________________ (a) Morphological properties: Item Pseudomonas DK-2 Arthrobacter DK-19 ______________________________________ Shape of cells rod-shaped rod-shaped Size of cells (.mu.) 1.5 .times. 0.8 2.5 .times. 0.8 Pleiomorphism none Mixture of rods of ordinary curved line- like ones, V-shaped ones and stick-like ones Mobility positive positive: monotrichous very active peritrichous Spore none none Gram-staining negative having gram-positive granules within the gram-negative cells Acid-fastness none none ______________________________________
______________________________________ (b) Growth state in various culture media: Item Pseudomonas DK-2 Arthrobacter DK-19 ______________________________________ Plate culture semi-transparent semi-transparent medium of agar circular colonies cream-coloured circu- with bouillon with convex-circu- lar colonies with lar protuberance,. convex-circular pro- lustrous tuberance, lustrous Slunt culture of growth moderate, growth moderate, semi- agar with semi-transparent transparent thread- bouillon and lustrous like colonies with cream-like colour, lustrous Liquid culture growth moderate growth favorable medium with flocculent bouillon Stab culture with growth favorable growth favorable on gelatin and on the surface of the surface of the bouillon the culture medium culture medium as filiform Litmus-milk change to alkaline discoloration without color liquefying ______________________________________
______________________________________ (c) Physiological properties: Item Pseudomonas DK-2 Arthrobacter DK-19 ______________________________________ Reduction of nitrate + - Denitrification + + MR test - - VP test - - Production of indole - - Production of H.sub.2 S +(weak) +(weak) Hydrolysis of starch - - Utilization of citric acid in Koser's medium - + Christensen's - + medium Utilization of inorg. nitrogen source nitrate - - ammonium salt - - Production of dye - - Activity as urease - - Activity as oxidase + - Activity as catalase + + Range of growth pH 6 to 9.5 4.5 to 9.5, preferably 8 to 8.5 temperature (.degree.C.) 5 to 50 15 to 38, preferably 28 to 30 Aerobism yes yes O-F test (method of oxidative oxidative Hush Leifson) Production of acid acid gas acid gas and gas from sugar L-arabinose - - - - D-xylose - - - - D-glucose - - - - D-mannose - - - - D-fructose - - - - D-galactose - - - - Maltose - - - - Sacrose - - - - Lactose - - - - Trehalose - - - - D-solbitol - - - - D-mannitol - - - - Inositol - - - - Glycerol +(weak) - - - Starch - - - - Raffinose - - - - Inulin - - - - D-ribose - - - - Dulcitol - - - - Sorbose - - - - Carboxymethyl- - - - - cellulose Halotolerance in 5% by weight of does grow does grow aqeuous solution of sodium chloride 10% by weight of does not grow does grow slightly aqueous solution of sodium chloride Decompositive -- -- activity to gelatine Activity as -- -- DNA-ase Essential vitamines thiamine and pantothenic acid folic acid and nicotinic acid Source of isolation soil soil ______________________________________
On classifying these two strains on the ground of the bacteriological properties while referring to "Manual of Determinative Bacteriology, 8th Ed. (1974)" by Burgey, the strain DK-2 was identified to belong to the genus Pseudomonas, because it is a gram-negative rod which is monotrichous, positive in oxidase activity and positive in denitrification.
On the other hand, the strain DK-19 was identified to belong to the genus Arthrobacter, because it is a weakly grampositive rod having a pleiomorphism and is peritrichous and impossible to utilize saccharide.
The enzyme having both the activity of D-threonine-aldolase and the activity of D-allothreonine-aldolase according to the present invention can be produced by culturing, for instance, one of the strains in a nutrient culture medium which may be the same as those for culturing ordinarily any strain of bacterial containing saccharide such as glucose, glycerol, molasses and the like or organic carboxylic acid such as acetic acid, malic acid and the like as a carbon source, ammonium sulfate, ammonium chloride, urea and the like as a nitrogen source, yeast extract, pepton, meat extract, corn-steep liquor, etc. As an organic nutrient and magnesium, iron, manganese, potassium, phosphate, etc. as inorganic ion. The cultivation may be effected under the conventional conditions, that is, at a pH of 4 to 10 of the culture medium, at a temperature of 20.degree. to 60.degree. C. for 1 to 3 days of aerobic cultivation after being inoculated.
By culturing one of the strain under the conditions, the enzyme having both the activity of D-threonine-aldolase and the activity of D-allothreonine-aldolase is produced in the bacterial bodies and accumulated therewithin. In order to isolate the enzyme in a purer state from the cultured medium thereof, the proliferated bodies of the strain of microorganism (hereinafter referred to as "the bacterial cells") are destroyed by a known method such as a mechanical method, a treatment with an enzyme and an autolysing method to obtain a crude extract of the enzyme and then the crude extract was subjected to purification by a suitable combination of precipitation with ammonium sulfate or an organic solvent such as acetone or methanol, and chromatography while using an ion-exchanger such as diethylaminoethyl (hereinafter referred to as "DEAE")cephalose, DEAE-cephadex and calcium phosphate gel, etc. or an adsorbent, etc. In order to obtain the manifastation the enzymic activity thereof, the presence of a coenzyme, pyridoxal-5'-phosphate, is necessary in its reaction in an ordinary amount of 10.sup.-5 to 10.sup.-3 M.
Physico-chemical properties of the novel enzyme according to the present invention are explained as follows.
(1) Activity and substrate-specificity:
The novel enzyme according to the present invention decomposes both D-threonine and D-allothreonine into glycine and acetaldehyde, and on the other hand, does not act at all on L-threonine and L-allothreonine.
(2) Optimum pH: PA0 (3) pH range in which the novel enzyme is stable: PA0 (4) Method for determination of the enzymic activity: PA0 (5) Range of the optimum temperature for the activity: PA0 (7) Conditions of in-activation of the novel enzyme: PA0 (8) Agents inhibiting, activating or stabilizing the activity: PA0 (9) Coenzyme: PA0 (10) Molecular weight: PA0 (11) Elementary analytical composition: PA0 (a) Morphological properties: PA0 (b) Growth state in various culture media:
From the result of determination of aldehyde produced by the novel enzyme from D-threonine as a substrate at 30.degree. C. for 10 min. at one of a series of pH, it is found that the optimum pH of the novel enzyme was in a range of 7 to 9. The respective buffer solutions used are a 0.1M phosphate buffer for a range of pH of 4 to 7.5, a 0.1M tris-HCl buffer for a range of pH of 7 to 9 and a 0.1M sodium carbonate buffer for a range of 9 to 11.
From the result of determination of the remaining activity of the novel enzyme after heating a solution of the enzyme for one hour at 30.degree. C. at one of a series of pH, the pH range in which the novel enzyme can exist in a stable state is 6 to 9. The respective buffer solution used in the calturing are a 1.0M phosphate buffer for a range of pH of 4 to 7.5, a 0.1M tris-HCl buffer for a range of pH of 7 to 9 and a 0.1M sodium carbonate buffer for a range of 9 to 11.
The amount of acetaldehyde formed when 0.1 ml of a liquid containing the novel enzyme is added to 0.4 ml of a 0.1M tris-HCl buffer solution containing 100 micromols of D-threonine at pH of 8.0 and the mixture is heated at 30.degree. C. for 10 min is determined by the method of Paz (refer to Arch.Biochem.Biophys., Vol. 109, page 548(1965)), and the enzymic activity on decomposing 1 micromol of D-threonine at 30.degree. C. is taken as a standard, i.e., one unit (U).
From the determination of the amount of acetaldehyde produced by the novel enzyme under the conditions of the optimum pH (8.0) at one of a series of temperatures for 10 min. while using a 0.1M tris-HCl buffer solution, it is found that the range of the optimum temperature for the enzyme was 40.degree. to 50.degree. C.
(6) Heat-stability of the novel enzyme:
From the determination of the remaining activity after heating a solution of the novel enzyme in a 0.1M tris-HCl buffer solution at pH of 8.0 for one hour at one of a series of temperatures, it is found that the temperature at which the enzyme is stable was below 40.degree. C.
The novel enzyme according to the present invention is in-activated completely at a pH below 5 and over 11, and also completely in-activated after heating for one hour at a temperature over 70.degree. C.
The novel enzyme is activated and stabilized by mercaptoethanol, sodium sulfite, sodium hydrogen sulfite, dithiothreitol, and Mn.sup.2+, Co.sup.2+, Fe.sup.2+ or Mg.sup.2+, and on the other hand, the activity thereof is inhibited by monovalent Ag.sup.+, Cu.sup.2+, Hg.sup.2+, Zn.sup.2+, Pd.sup.2+, hydroxylamine and p-chloromercuribenzoate.
The coenzyme of the enzyme is pyridoxal-5'-phosphate.
The molecular weight of the enzyme is in a range of 100,000-150,000 as a result of gel-filtration by Cephadex.RTM. G-200.
50.7-52.7% of carbon, PA1 6.8-8.8% of hydrogen and PA1 14.7-16.7% of nitrogen PA1 (1) Shape and size of the cells: rod-shaped of 0.8=2.0 micrometers, PA1 (2) Pleiomorphism: none, PA1 (3) Mobility: positive, multitrichous, PA1 (4) Spore: ellipsoidal in shape existing in the position out from the center, PA1 (5) Gram-staining: positive, PA1 (6) Acid-fastness: none. PA1 (1) Plate culture medium of agar with bouillon: favorable with circular colonies, PA1 (2) Slunt culture of agar with bouillon: favorable, semitransparent with luster, PA1 (3) Liquid culture medium with bouillon: favorable, PA1 (4) Stab culture with gelatin and bouillon: favorable filiform in the surface of the culture medium, PA1 (5) Litmus-milk: decolorated and liquefied. PA1 (1) Optimum pH; 8 to 9, PA1 (2) Optimum temperature: 60.degree. to 70.degree. C., PA1 (3) Conditions for inactivation: inactivated within one hour at 30.degree. C. and pH of 5 to 11 or in one hour at a temperature of higher than 50.degree. C. and pH of 8, PA1 (4) Inhibitants: Cu.sup.2+, Hg.sup.2+ and Ag.sup.1+, PA1 (5) Stabilizers: mercaptoethanol, dithiothreitol and sodium sulfite, PA1 (6) Coenzyme: pyridoxal-5'-phosphate. PA1 (7) Molecular weight: 100,000 to 150,000 according to the determination of gel-filtration by Cephadex.RTM.G-200. PA1 (8) Elementary analytical data:
Since the known threonine-aldolase and allothreonine-aldolase decompose only L-threonine and L-allothreonine respectively, and those decomposing the D-isomer have never been known, each enzyme found in the strains respectively is a novel enzyme having a new activity.
Namely, any anzyme have D-threonine-aldolase activity and can be used in the process of the present invention as far as the enzyme can decompose both D-threonine and D-allothreonine to convert them into glycine and acetaldehyde.
L-allothreonine-aldolase used in the process according to the present invention is an enzyme catalyzing L-allothreonine to convert thereof into glycine and acetaldehyde, and the presence thereof has been known in the sheep liver and corn seed in germination, however, the microbiological production thereof has never been known.
However, the present inventors have found out that some bacteria respectively belonging to the genera Bacillus, Pseudomonas, Arthrobacter and Alcaligenes produce the enzyme, and have found the method for producing the enzyme industrially in a large scale. As the examples of the microorganism having the productivity of L-allothreonine-aldolase, a strain of Bacillus DK-315 (deposited in Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan under a deposite number FERM-P No. 6202) isolated from a soil, a strain of Arthrobacter DK-19 (FERM-P No. 6201), the strain of Pseudomonas DK-2 (FERM-P No. 6200) and a strain of Alcaligenes faecalis (deposited in Institute for Fermentation Osaka, Japan under a deposite number of IFO-12669) can be used.
The bacteriological properties of the strain of Bacillus DK-315 (FERM-P No. 6202) are as follows.
______________________________________ (c) Physiological properties: ______________________________________ (1) Reduction of nitrate: + (2) Denitrification: + (3) MR test: + (weak) (4) VP test: - (5) Production of indol: - (6) Production of H.sub.2 S: - (7) Hydrolysis of starch: + (8) Utilization of citric acid in Koser's medium: - in Christensen's - medium: (9) Utilization of inorganic nitrogen nitrate: - ammonium salt: + (10) Production of dye: - (11) Activity as urease: - (12) Activity as oxidase: + (13) Activity as catalase: + (weak) (14) Range of growth pH: 5 to 12 temperature: 5 to 40.degree. C. (15) Aerobism: yes (16) O-F test (Hush F Leifson's method): (17) Production of acid (acid) (gas) and gas from sugars: L-arabinose - - D-xylose - - D-glucose + + D-mannose + + D-fructose + + D-galactose + + Maltose + - Sucrose - - Lactose - - Trehalose + - D-sorbitol - - D-mannitol + - Inositol - - Glycerol + + Starch + - (18) Halotrelance in an does not aqueous 5% sodium grow chloride solution: (19) Decompositive + activity to gelatine: (20) Activity as + DNA-ase: (21) Essential vitamins: none ______________________________________
From the above-mentioned bacteriological properties, while referring to "Manual of Determinative Bacteriology, 8th Ed(1974) by Burgey, the strain DK-315 was identified to belong to the genus Bacillus because of moving by peritricha and of gram-positive rod having a capacity of forming spores.
L-allothreonine-aldolase can be produced by culturing each of the above-mentioned strains in a nutrient culture medium which is usually used for culturing a bacterial strain containing a saccharide such as glucose, glycerol and molasses or an organic acid such as acetic acid, malic acid and the like as a carbon source, ammonium sulfate, ammonium chloride, urea and the like as a nitrogen source, and an inorganic ion such as ammonium sulfate, ammonium chloride and urea, as an organic nutrient source of yeast-extract, peptone, meat-extract, corn-steep liquor and the like and a metal salt such as magnesium, iron, manganese, potassium and phosphates usually according to the conventional method of cultivating bacteria at a pH of 4 to 10 for one to three days at 20.degree. to 60.degree. C. aerobically at the pH of the culture medium of 4.0 to 10.0.
Thus, L-allothreonine-aldolase is produced and accumulates in the bacterial bodies, and accordingly, in the case of isolating the enzyme from the cultured medium, the bacterial cells are broken by a known method such as a mechanical means, an enzymic means or a autolysing method and then the crude extract of the enzyme is prepared. The crude extract was treated by a suitable combination of precipitation with ammonium sulfate or a solvent such as acetone or ethanol and chromatography while using ion-exchangers such as DEAE-cepharose, DEAE-cephadex, gel of calcium phosphate or adsorbents to be the enzyme product of a high quality. The followings are the simple physico-chemical properties of the thus purely obtained L-allothreonine-aldolase.
Since L-allothreonine-aldolase requires pyridoxal-5'-phosphate as a coenzyme for exhibiting its activity, usually 10.sup.-3 to 10.sup.-5 M of pyridoxal-5'-phosphate is made to coexist with the enzyme whenever it is reacted. PA2 C: 51.4-53.4% PA2 H: 6.5-8.5% and PA2 N: 14.2-16.2%.
L-allothreonine-aldolase used in the present invention is enough for the purpose if it decomposes L-allothreonine into glycine and acetaldehyde, and it goes without saying that the enzyme is not restricted to that derived from microorganism.
The enzymes used in the present invention, that is, D-threonine-aldolase and L-allothreonine-aldolase, are respectively enough for the purpose in the case where they are respectively under the condition of capable of exhibiting the enzymatic activity thereof, and they are never restricted under isolated condition, and accordingly, half-purified products, crude extract, moreover, the cultured medium, living cells, freeze-dried cells, dried cells by acetone, ground cells, ground sheep liver and the like may be used as it is. The immobilized enzyme or the immobilized cells by a known means can be used. As a method of immobilization, a method of combining with a carrier, a method of cross-linking, a method of entrapping, a method of agglutination and the like are broadly usable.
DL-threonine or a mixture of DL-threonine and DL-allothreonine may be the product obtained by any known method, and for instance, a method wherein an acetoacetate is used as the starting material for obtaining an ester of alpha-amino-beta-hydroxybutyric acid and the amino-hydroxybutyrate is reacted with thionyl chloride to form an oxazoline-ester, and then the ester is hydrolyzed into the product by heating (refer to J.Am.Chem.Soc., 71, 1101(1949)) and a method wherein vinyl acetate as the starting material is subjected to hydroformylation by the oxo process to give alpha-acetoxypropionaldehyde and the aldehyde is reacted with hydrogen cyanide and ammonia to be alpha-amino-beta-hydroxybutyronitrile which is in turn reacted with phosgen to be a derivative of oxazolidone and then the derivative is subjected to hydrolysis to be the product (refer to Japanese Patent Publication No. 40-11608(1965)) are utilizable for the purpose. However, since in the process according to the present invention, the object product is L-threonine and by-product consists of glycine and acetoaldehyde, a synthetic method which produces the allo-forms in a smaller amount and requires glycine and acetaldehyde as the starting compounds is preferable. In this connection, a method wherein a metal salt is reacted with glycine to be a metal complex of glycine and then acetaldehyde is condensed with the metal complex of glycine (refer to Japanese Patent Publications Nos. 36-19562(1961) and 47-39093(1972)) is the suitable method for producing a mixture of DL-threonine and DL-allothreonine. The solution containing the mixture may be an aqueous solution which is obtained by dissolving the once-obtained product as crystals into water, however, it may be the liquid obtained by the synthesis, or may be any intermediate liquid in the course of obtaining the crystals of the mixture. In short, the liquid containing DL-threonine and DL-allothreonine used in the process according to the present invention may be a solution containing DL-threonine and DL-allothreonine, and the ratio of D-isomer to L-isomer in the solution and the ratio of threonines to allothreonines in the solution are out of the question. Further, the solution may contain any other impurities in addition to DL-threonine and DL-allothreonine. Any impurities inhibiting the enzyme reaction, if any, should be removed in advance. For instance, in the case where a metal ion used in the synthesis is deleterious in the enzyme reaction, it should be removed by a cation-exchanging resin, etc. in advance of the enzyme reaction.
There is no particular difficulty in treating (catalyzing) a solution containing DL-threonine with D-threonine-aldolase or treating (catalyzing) a solution containing both DL-threonine and DL-allothreonine with the mixture of D-threonine-aldolase and L-allothreonine-aldolase. In short, the indicated enzyme may be made present in the aqueous solution of the indicated substance. The concentration of DL-threonine and/or DL-allothreonine in the solution may be an extent which does not remarkably inhibit the enzymatic activity, and it is preferably 0.1 and 2 mol/liter. The solvent of the enzyme reaction system is, in principle, water, however, an organic solvent may be contained if it does not inhibit the enzyme reaction. Although the pH of the reaction system depends on the enzyme, it is preferably around pH 7 to 10 in the cases where the enzyme is obtained in Examples described. Although the reaction temperature depends on the enzyme, it is preferably 30.degree. to 45.degree. C. in the case of using the enzyme prepared in Examples described. In addition, in the case of using the enzyme prepared in Example, the enzyme reaction can be accelerated by bringing 10.sup.-3 to 10.sup.-5 molar amount of pyridoxal-5'-phosphate coexist in the system as a coenzyme. Furthermore, a surfactant may be added to the reaction system for various purposes. The enzyme reaction can be carried out by a batch system or in a continuous system. D-threonine-aldolase and L-allothreonine-aldolase may be added together with or separately.
The reaction time can be selected optionally according to the purpose of L-threonine for use. For example, in the case where L-threonine in which the existence of undecomposed isomer is allowable is to be obtained, the reaction may be stopped before the completion of enzymatic decomposition of the isomer. At any rate, a reaction time of 5 to 100 hours is sufficient for every enzyme.
After the enzyme reaction finishes, the suspending matters in the reaction mixture are removed by centrifugation or filtration, if necessary, and the obtained reaction mixture is purified by treatment of ion-exchanging resin and crystallization, and after decolorizing the reaction solution by activated carbon, etc., the decolorized solution is condensed to obtain the crystals of L-threonine in a pure state. The reaction product other than L-threonine comprises glycine and acetaldehyde in the case of using D-threonine-aldolase and also in the case of using D-threonine-aldolase and L-allothreonine-aldolase, and glycine can be separated and isolated by chromatography, for example, using ion-exchanging resin. Because of the non-enzymatic condensation of acetaldehyde with glycine during the enzyme reaction or of the re-combination with glycine by each enzyme, it may be better to recover acetaldehyde during the enzyme reaction by distillation, etc.
According to the process of the present invention, the removal of D-threonine or DL-allothreonine from threonine mixture which has been difficult can be carried out by one convenient and simple step. Accordingly, the present invention has dissolved the large problem which has hindered the separation of the isomer and has provided with the method for producing L-threonine at a low price from threonine which is synthetically produced. Particularly, when combined with the synthetic method for producing threonine from glycine and acetaldehyde, it is more preferable, because the by-products of the enzyme reaction can be re-used as the starting materials.